1. Field of the Invention
The invention relates to cryogenic liquid pumps. More particularly, it relates to a reciprocating-type pump for cryogenic liquids.
2. Description of the Prior Art
The pumping of cryogenic liquids is confronted with problems in heat management and materials selection in light of the low temperatures involved in such operations. Heat conduction from the warm end of a cryogenic pump to the pumping chamber portion of the pump body, heat in-leakage from the ambient environment, frictional heat generated by the reciprocating motion of the plunger, and heat generation in the pumping chamber due to fluid compression have long been recognized as major sources of pump inefficiency.
The principal prior art approach for overcoming such problems has been to endeavor to intercept the heat conducted from the warm end of the pump by means of heat exchange with a cold fluid. Thus, pumps utilizing suction liquid, blowby fluid and pressurized liquid as a cold heat fluid have been proposed in the art. For example, Picard, U.S. Pat. No. 1,895,295, described the submersion of the pump in a cryogenic liquid and the use of heat transfer fins on the pump body to improve heat transfer between the pumping chamber and the pump liquid. Similarly, Hughes, U.S. Pat. No. 2,931,313, and Lady, U.S. Pat. No. 2,973,629, provide an annular cooling jacket surrounding the pumping chamber, with cryogenic liquid being passed through said cooling jacket prior to being introduced into the pumping chamber on a suction stroke of the pump. In the Riede, U.S. Pat. No. 2,730,957, Gottzmann, U.S. Pat. No. 3,136,136 and Schuck, U.S. Pat. No. 4,156,584, patents, blowby fluid is passed in a direction opposite to the heat flux so as to intercept the heat conducted from the warm end of the pump. While such approaches can effectively prevent major problems, such as vapor binding, which would normally accompany an inordinate heat flux to the cold end of a pump, there are disadvantages associated therewith. Most importantly, it is not feasible to precisely control the amount of cooling being accomplished. In many instances, therefore, the warm end of the pump may actually become too cold for proper packing performance. Frost may actually form in some cases and destroy the packing. As a result, auxiliary heating means may have to be employed in many instances to enable continued trouble-free operation under a range of operating conditions. Such a heating requirement represents an additional and otherwise unnecessary heat load in the pump.
In addition to such efforts to prevent the conduction of heat from the warm to the cold end of cryogenic pumps, the prior art has also endeavored to control such heat conduction by appropriate structural means. In both the Riede and the Schuck patents referred to above, for example, a thin tubular section is employed to connect the cold pumping chamber to the warm packing end of the pump. The heat flux from the warm end of the pump was said to be minimized by making such tubular section as long and as thin as is consistent with adequate structural strength. In the designs shown in both patents, the thickness of the pump body is increased in the pumping chamber portion of the pump as is necessary to withstand the pressure loading, i.e. the hoop stress, on this portion of the pump. While such an approach provides some improvement in the art of cryogenic pump design, it will be appreciated that the degree to which the thickness of the thin tubular section can be reduced is constrained by the tensile loads developed during operation of the pump. The Riede and Schuck patents, for example, teach that the tubular section must, in combination with the thicker pumping chamber portion of the pump body, bear the tensile and hoop stresses created by the pumping pressure. Accordingly, the thickness of the tubular section must be sufficient to support this tensile load although a thinner construction would otherwise be desirable for purposes of minimizing the heat flux from the warm end to the cold end of the pump.
A particularly desirable approach to such heat management problems is disclosed in U.S. patent application Ser. No. 202,476, entitled "Cryogenic Reciprocating Pump," filed Oct. 31, 1980. As disclosed therein, the pump body has a flange at its forward end that is integral with the cylindrical shell. The shell is spaced apart from the pump body so as to form an annular insulation space surrounding the pump body. The shell, at its opposite end, is attached to another flange positioned at the base of the pump's packing assembly. The pump body is rigidly secured to a suitable power frame by still another flange that is affixed to the cylindrical shell. The rearward end of the pump body is coupled to the packing assembly by a thin corrugated metal member that is capable of minor axial adjustment under the influence of external deforming forces, e.g. such forces caused by thermal expansion and contraction of the pump body. The packing assembly is actually supported by the cylindrical shell through its mounting on the flange affixed to said shell. The latter approach serves to remove the tensile stresses that could otherwise exist in the pump body as a result of axial temperature differences and the end load produced by the pump pressure. As a result, it is possible to achieve desirable reductions in the size and weight of the pump body. In addition, this approach also serves to significantly reduce the axial heat conduction from the warm end to the cold end of the pump since the thin corrugated metal member that constitutes the only direct thermal link between the two ends represents a significant impediment to heat transfer from the warm end to the cold end of the pump.
In terms of thermally isolating the pump body portion from the drive frame, the general practice in the cryogenic pump art, including the design of said Serial Number referred to above, has been to engineer the pump body as an element distinct from the power frame. the pump body has been coupled to a mounting plate on the power frame by means of a suitable mounting flange. The pump assembly also normally employs a spacer element interposed between said mounting flange and the mounting plate for purposes of achieving the desired goal of thermally isolating the body portion of the pump from the drive frame. In any such pump design, therefore, each pump body section must be separately attached to the power frame and separately aligned with the centerline of the crosshead thereof. It will be appreciated that the proper positioning of the various components and the ultimate alignment of the overall pump assembly will be complicated as a result of the normal machining tolerances pertaining in the production of the various component parts. Moreover, the independent nature of the two major pump components tends to compound the problem of assembly alignment. In a triplex pump, for example, three pumping sections must be individually mounted on the power frame. Consequently, there are three opportunities for introducing an assembly-related misalignment into the pump assembly. In the basic prior art designs for cryogenic pumps, therefore, some element of misalignment must be tolerated. This misalignment serves to reduce the reliability and operating life of various pump components, such as the rider rings, piston rings, packing seal rings, and crosshead bearings. Furthermore, individual support of the multiple pumps will not provide the same rigidity as a unitary support under comparable weight and space limitations.
Because of the circumstances discussed above, it will be appreciated that there is a continuing desire to achieve further developments in the field of cryogenic pumps. In particular, there is a need and desire for the development of reciprocating-type cryogenic pumps capable of pumping cryogenic liquids at high operating pressures, i.e. at pressures in excess of about 500 psi. Such pumps are desired for operation with minimum heat leaks and cooldown losses, and with simplified pump construction and serviceability, together with improved pump reliability.
It is an object of the invention, therefore, to provide an improved cryogenic liquid pump.
It is another object of the invention to provide a reciprocating-type cryogenic pump suitable for high pressure operation.
It is another object of the invention to provide a cryogenic liquid pump having improved performance reliability and operating life.
It is a further object of the invention to provide an improved cryogenic pump capable of minimizing heat leaks and cooldown losses.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.